1. Field of the Invention
This invention relates to photochromic materials having a thin photochromic film and also to a method for fabricating the photochromic material.
2. Description of the Prior Art
Photochromic materials are known as substances which undergo reversible color changes by irradiation of light. A number of organic photochromic substances are known as undergoing photoreactions such as cis-trans isomerization, photo-ring-cleavage reactions, photodimerization and the like.
These photochromic materials or substances have wide utility in the fields of glasses whose color is changed by exposure to UV rays, photo-switching devices wherein changes in the refractive index or absorption of light are utilized, or erasable optical memories, and the like. Photochromic substances have been frequently employed in the form of a thin film formed on a support. For the formation of the thin film, many techniques have been developed and used, including casting techniques, dispersion-in-polymer casting techniques wherein the substances are dispersed in polymers, vacuum deposition techniques, the Langmuir-Blodgett technique (hereinafter referred to simply as LB technique or method), and the like.
When the casting, dispersion-in-polymer casting and vacuum deposition techniques are used to make a thin film from photochromic substances which exhibit a good photochromic reaction in a solution, a stable photochromic reaction may not occur in the thin film. The reasons for this are considered as follows: (1) the photochromic substance is crystallized in the thin film; (2) the photochromic substance is in such an associated state that is unlikely to undergo the photochromic reaction; and (3) the photochromic substance is not dispersed uniformly, so that a homogeneous film is not obtained. The LB method is advantageous over the above techniques in that since a monomolecular film of a good quality is formed at the interface between the gas and the liquid, the above problems are not likely to be raised. However, limitation is placed on the LB method. More particularly, in order to form a monomolecular film at the interface, the photochromic substance should have surface activity and a long-alkyl chain. This means that both photochromic substances having no long-chain hydrocarbon group and water-soluble photochromic substances cannot be applied by the LB method.